The Cubs made the makeup date with the Brewers for the April 27 rainout official this morning. It had previously been announced by the Brewers official Twitter account earlier this week as a split doubleheader on August 16.
They also announced the makeup date for the Braves rainout on April 30:
This was the only date that made logical sense; the Cubs are completing a short three-game homestand against the Reds with an off day July 7, and the Braves had an off day on that date before coming to Chicago to play the White Sox.
These games are now listed on the Cubs schedule on their website, and tickets are available. The Brewers game probably has more tickets available than the Braves game, as the latter was expected to be a sellout. If you already had a ticket for either of the rainouts, you can use it on the makeup date.
Finally, the Cubs announced that the Ron Santo replica statues that were supposed to be given out before the game April 30, will be handed out to "up to" the first 10,000 fans arriving for the game on July 7.